Optimal maturation depends on a programmed unfolding of structural changes and biochemical competence which to some degree, in man, are dependent upon a supply of thyroid hormones. We have previously shown that breast feeding is of major importance in decreasing the severity of congenital hypothyroidism in the absence of early diagnosis and treatment. Furthermore, we have laboratory confirmation of the thyroid hormone content of breast milk. From preliminary investigations we have noted that straight forward determination of triiodothyronine (T3) and thyroxine (T4) in milk is not reliable because of a number of different factors which are influenced by the manner in which milk is preliminarily handled and ultimately assayed. Milk T3 and T4, following extraction, do not behave as extracted serum T3 and T4 on LH-20 columns. Whereas the serum radioimmunoassayable T3 and T4 fractions are eluted in the same fractions in which their respective added labeled tracers elute, all radioimmunoassayable material in milk is found in the void volume, associated with a moiety which absorbs at OD280. Based on these results, we are proposing to look into the chemical and biological characteristics of the radioimmunoassayable forms of thyroid hormones in milk and the nature of the "carrier" moiety(protein?) which has been detected. The objective of the present proposal would be directed to better understand, in physiological terms, the thyroid link between lactating mother and suckling infant by establishing (a) the true concentrations of T3 and T4 in milk, temporal variations of their levels with lactation and the chemical identity of the radioimmunoassayable material with native T3 and T4, and (b) the nature and biological activity or significance of the "carrier" moiety. The approach will involve RIA measurements of T3, T4 and rat milk and human milk obtained from "human milk banks". We will employ a rat model for preliminary studies dealing with the "carrier" fraction after it has been labeled both in vivo (125I) and in vitro. Comparison of the behaviour of these proteins by various physicochemical means with one another and with in vitro iodination of human "carrier" will enable us to isolate and obtain enough material to characterize its properties and biological effectiveness.